The light scattering measuring apparatus is used as an optical measuring apparatus in which a laser light is irradiated to colloid particles in a fluid, blood corpuscles in blood or the like and the Doppler shift of the scattered light is observed, thereby to measure the moving velocity of the particles.
As the light scattering measuring apparatus of the type above-mentioned, a heterodyne-type light scattering measuring apparatus is widely used in which the scattering light is heterodyne-detected with the use of a reference light obtained by splitting light from the same light source, so that the frequency distribution of the scattered light is detected with precision.
In the heterodyne-type light scattering measuring apparatus, it is preferable to make a provision such that the light scattering measuring angle is continuously changed. This is because, in such an arrangement, particles having a variety of sizes may be efficiently measured and the velocity distribution of particles having a size distribution may be measured according to the scattering angle. Accordingly, a variety of light scattering measuring apparatus of this type have been proposed.
For example, there has been proposed (1) apparatus, as shown in FIG. 6, in which the incident light LD and the reference light LR parallel thereto are formed from a laser beam L with the use of a half-mirror BS and a reflecting mirror P, and both lights are incident upon a large-diameter convex lens LE, and the scattering light LS from a sample S at the focal position F of the convex lens LE is measured together with the reference light LR (E. E. Uzgiris, D. H. Cluxton, Rev. Sci. Instrum, 51 (1), Jan. 1980).
In the light scattering measuring apparatus above-mentioned, the Doppler shift may be observed according to the heterodyne method. However, to change the scattering angle .theta., it is required to adjust the incident position of the reference light LR with the use of the general nature that the refraction angle of light incident upon a convex lens becomes greater as the incident point is separated away from the center of the convex lens. It is therefore necessary to move the reflecting mirror P to change the distance X between the direct light LD and the reference light LR. In addition to such a movement of the reflecting mirror P, the rotation of the position of a photodetector DET for receiving the scattering light LS, around the focal point F of the lens LE is also required.
However, even though the reflecting mirror P is moved, the scattering angle .theta. cannot be directly measure since the optical path is not visual. Accordingly, the scattering angle .theta. may be merely presumed based on a calculating equation representing the relationship between the distance X and the scattering angle .theta., or based on the angle at which the light intensity at the photodetector DET is maximized, or the like. This involves the likelihood that an error will be introduced. Further, since the reference light LR passes through the sample, the noise component in the signal is increased. This makes it difficult to measure small-diameter particles of which scattering intensity is small. Thus, the resolution is disadvantageously deteriorated.
There has been also proposed an apparatus, as shown in FIG. 7, in which only the incident light LD is incident upon a sample S through a convex lens LE1, and the resultant scattering light LS is united, at a half-mirror BS, with the reference light LR guided from another optical path, and the heterodyned light is incident upon a photodetector DET, where measurement is carried out. According to this arrangement, measurement similar to that achieved in the apparatus shown in FIG. 6 may be made by integrally moving the sample S and the lenses LE1, LE2 along a direction A. However, it is also difficult, likewise in the apparatus shown in FIG. 6, to directly measure the scattering angle .theta..
There has been also proposed a light scattering measuring apparatus in which a reference light is guided along the rotary shaft for rotating a sample stand and the scattering angle may be changed only by rotating the sample stand (P. J. Nash, T. A. King, J.Phys. E:Sci. Instrum., Vol. 18, 1985). As shown in FIG. 8, this light scattering measuring apparatus has a rotary stand T rotatable around a vertical axis of rotation A. Disposed on the rotary stand T are a housing chamber SH for housing a sample S, prisms P5, P7, a half-mirror BS2, pinholes PH, a lens LE, a polarizer PL and a photodetector DET. These members on the rotary stand T are secured thereto in an integrally rotatable manner therewith. The housing chamber SH and the prism P5 are disposed as intersecting the axis of rotation A. Under the rotary stand T, a prism P3 is disposed as intersecting the axis of rotation A.
Of the rays of light emitted from a laser L, the reference light LR reflected by a half-mirror BS1 is guided under the rotary stand and then reflected by the prism P3. The reference light LR reaches the upper portion of the rotary stand T along the axis of rotation A. The reference light LR is then reflected by the prism P5 and advances horizontally. After being reflected by the prism P7, the reference light LR is incident upon the half-mirror BS2. On the other hand, the incident light LD having passed through the half-mirror BS1, is reflected by prisms P2, P4 and passes through the sample housing chamber SH. Then, the scattered light LD is incident upon the half-mirror BS2. After spatial mixing at the half-mirror BS2, the reference light LR and the scattered light LD are incident upon the photodetector DET.
According to the light scattering measuring apparatus having the arrangement above-mentioned, when a portion of the optical path of the reference light LR is identical with the axis of rotation A, the detection angle, i.e., scattering angle, of the direct light LD with respect to the sample, may be freely set merely by rotating the rotary stand T.
In the light scattering measuring apparatus above-mentioned, however, the polaraization of the reference light LR with respect to the rotary stand T is changed with the rotation of the rotary stand T. More specifically, the polarization of the reference light LR guided along the axis of rotation A is changed in direction according to the rotation angle .theta. of the rotary stand T when viewed from the coordinate system which rotates together with the rotary stand T. Accordingly, when the reference light LR is incident upon the half-mirror BS2 through the prisms P5, P7 secured to the rotary stand T, the reference light LR presents an angle of polarization corresponding to the rotation angle .theta. of the rotary stand T. For example, this will be discussed with reference to FIG. 9 (a) and (b). It is now supposed that the light emitted from the laser L presents a direction of polarization parallel to the plane of the sheet of the drawing as shown in FIG. 9. In FIG. 9 (a), the light spatially mixed at the beam splitter BS2 presents the same direction of polarization Y. When the rotary stand T is rotated with 8 set to 90.degree., the prism P5 is also rotated at 90.degree.. The reference light LR emitted from the prism P5 in a direction vertical with respect to the drawing plane presents a direction of polarization Z parallel to the drawing plane, as shown in FIG. 9 (b). Accordingly, the lights to be united at the half-mirror BS2 are polarized at a right angle to each other. This decreases the intensity of the signal obtained through a polarizer PL.
To solve such a problem, the author of this document has proposed to interpose a quarter-wave plate in the optical path of the reference light LR to convert the linear polarized light into a circular polarized light, or to increase the power of the reference light LR to utilize the light deviating from the polarized light (generally, even a linear polarized light does not always form a perfectly linear polarized light, but contains some perpendicular component).
However, when the quarter-wave plate is interposed, this presents the problem that the number of component elements is increased. Alternatively, to increase the power of the reference light LR, a large-size laser device is required. This disadvantageously increases the size of the apparatus in its entirety.
In view of the problems above-mentioned, the present invention is proposed with the object of providing a light scattering measuring apparatus in which, without polarization of light being shifted, the scattering angle may be continuously and accurately set by a simple rotating operation and the angle thus set may be readily measured.